Stabilization of radio controlled aircraft

ABSTRACT

A stabilization system is disclosed in which stabilization pitch and roll signals effective for stabilizing the aircraft are combined with respective pilot provided elevator and aileron position demand signals in accordance with a function which reduces the effects of the stabilizing signals in dependence on increasing values of the respective elevator and aileron position demand signals.

This invention relates to stabilisation of the attitude of a radiocontrolled aircraft in pitch and roll.

Flying radio controlled model aircraft requires training and theacquisition of skill. Special easy to fly training aircraft are used fornovices in order to reduce the risks of a crash. Especially during earlytraining there is a risk that the novice pilot will create a situationwhich s/he has insufficient skill to recover from. Even training modelswill merely continue at the existing attitude if the controls areneutralised so positive action needs to be taken to recover. Radiofailure, or interference will, or can, lead to a neutral setting of thecontrols so that the model just continues as it was before theinterference. Even training models can be expensive to replace and acrash always carries an element of risk to personnel.

U.S. Pat. No. 2,828,930 describes a system for stabilising the attitudeof an aircraft. If the system were applied to a radio controlledaircraft, however, the stabilisation system would continually resistattempts by the pilot to control the aircraft's attitude to performmanoeuvres.

Against this background, in accordance with the invention, there isprovided a radio controlled aircraft, including a radio receiver forreceiving an elevator position demand signal for specifying a requiredposition for the elevator and at least one aileron position demandsignal for specifying required positions for the or one of the ailerons;stabiliser means for generating pitch and roll stabilisation signalsdependent on differences in the aircraft's attitude from level in pitchand roll respectively, mixer means for combining the stabilisation pitchand roll signals with the elevator and aileron position demand signalsin a sense to stabilise the aircraft in accordance with a function whichreduces the effect of the stabilisation signals in dependence onincreasing values of elevator and aileron position demand signalsrespectively; and control means for controlling the positions of theelevator and ailerons in accordance with respective combined signals.

Importantly, if the pilot removes all input by neutralising the positionof the stick, the stabilising signals will bring the aircraft into levelflight. Small stick inputs will still be resisted by the stabilisationsignals. However, large stick inputs will be less affected.

In one form, the values of the modified pitch and roll stabilisationsignals are proportional to those of the (unmodified) pitch and rollstabilisation signals below respective thresholds and equal to zeroabove said thresholds.

More preferably, the values of the pitch and roll stabilisation signalsare reduced in proportion to any increase in the respective elevator andaileron position demand signals from a maximum when the respectiveposition demand signal is zero, to zero when the respective positiondemand signal has a predetermined value larger than zero, e.g.corresponding to maximum deflection of the elevator or ailerons.

Most preferably, the radio receiver is adapted for receiving a gainsetting signal to which the means for modifying the pitch and rollstabilisation signals is responsive to provide a level of gain dependenton the gain setting signal.

In one convenient form, two pairs of directional light sensors areprovided, the sensors of each pair being responsive to light input fromtwo different directions on opposite sides of a respective axis toprovide respective light level signals indicative of the level of lightsensed said stabiliser means being responsive to difference between thelight level difference signals to provide difference signals.

The signal from each sensor will vary in value dependent on whether itis directed above or below the horizon. When the aircraft is level andthe two sensors are directed at the horizon, the difference should bezero.

The light sensors are preferably photo conductive light sensorsconnected in series across a reference voltage. The light sensors may bealigned approximately normal to respective pitch and roll axes.

In order to reduce the possibility of a sensor being incapacitated by adeposit of oil or other pollutant it would be preferable if none of thesensors faced directly in the direction of motion. In a preferredarrangement, the light sensors are aligned in directions approximatelybisecting the angles between the pitch and roll axes.

In this arrangement, in order to derive suitable stabilisation signalsthere is preferably included means to add the differences in order toprovide one of the pitch and roll stabilisation signals and to subtractone difference from the other in order to provide the other of the pitchand roll stabilisation signals.

One embodiment of the invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is an exploded pictorial view of a sensor unit for apparatusembodying the invention for stabilising the attitude of a craft;

FIG. 1A is a pictorial view of the sensor unit of FIG. 1 installed on amodel radio controlled aircraft; and

FIG. 2 is a block diagram of the apparatus.

Referring to the drawings, the apparatus is intended to stabilise theattitude of a model aircraft about two axes so as to control the pitchand the angle of bank of the aircraft. A sensor unit 2 shown in FIG. 1is has the external semblance of a model radome. The unit has a body 4and a cover 6. Four small holes 8 are provided in the periphery of thebody aligned with corresponding holes 10 on the cover. Two of the holes8,10 are aligned diametrically opposite on one axis A--A. The other twoholes 8,10 are aligned diametrically opposite on an axis B--B normal tothe axis A--A and generally in the same plane.

The body 4 has a central chamber 12 into which the holes 8 extend. Aphotoconductive light sensor (photo resistor) 14 is mounted in the innerend of each hole 8 so as to sense light entering from the outer end ofthe hole. In the particular example the holes are about three times aslong as their diameter, more preferably 2 to 21/2 times, so that eachphoto resistor responds to light from the general direction of the axisA--A or B--B. In an even more preferred example the outer end of theholes is not circular but wider in the horizontal plane than in thevertical plane. For example, the holes may be 4 mm diameter round at thesensors and 6 mm wide by 2.5 mm high at their outer ends. More light isthus collected from a wider arc of the horizon.

The photo resistors are more sensitive to the blue and ultra violet endof the spectrum than to the red and infra red, so that the backgroundillumination level normally increases above the horizon and reducesbelow the horizon. This is to avoid hot ground masking, orcounteracting, the increase of illumination level above the horizon.Either the photo resistors may be selected to be more sensitive to theblue/ultra violet end of the spectrum or, if light sensors are usedwhich are undesirably sensitive to the red/infra red end of thespectrum, filters may be used to filter out the undesired part of thespectrum.

The sensor unit 2 is installed on the aircraft so that when it is inflying in a level attitude the axes A--A and B--B are directed at thehorizon. If the aircraft's attitude is different from that, then one orboth of the axes A--A and B--B will no longer be aligned with thehorizon. Supposing the axis A--A is no longer aligned with the horizon,then one of the respective photo resistors will be directed above thehorizon and will receive more light than the other which will bedirected below the horizon.

A cable 16 and connector 18 (FIG. 1) connect the photo resistors asshown in FIG. 2. The two photo resistors 14 on one axis, e.g. A--A areconnected in series across a voltage supply. The node between the photoresistors is connected to signal processing means 19 at an analog todigital convertor 20 in which an analog to digital conversion isperformed. When the axis A--A is horizontal, the respective photoresistors 14 are directed at the horizon and receive nominally the samelevel of light so that the node between them is at a voltage level halfway between the supply rails. If the axis A--A is inclined in onedirection to the horizon, the upwardly directed photo resistor receivesmore light than the downwardly directed photo resistor. The resistanceof the upwardly directed photo resistor falls and that of the downwardlydirected photo resistor rises so that the voltage at the node betweenthem rises.

The voltage at the node between the two photo resistors therefore risesand is converted into a digital light level difference signal S1, S2 bythe analog to digital converter 20. The light level signal istransmitted to a signal shaping unit 22. With switches 24 set in theposition shown, the light level signals S1 and S2 are each inputdirectly with a digital reference to a subtractor 26. The digitalreference signal is equivalent to equal light being received by thephoto resistors. The output difference signal Rs represents angulardeflection of the aircraft about the roll axis and the output differencesignal Ps represents the angular deflection of the aircraft about thepitch axis.

The sensor unit 2 should be placed on the aircraft so that it isnominally horizontal in level flight. Provided the background lightlevel is uniform in a plane parallel to the horizon, the digitalreference signal could be set permanently. However, in reality, thebackground light level will not be uniform and the sensor unit 2 may notbe completely accurately aligned. To accommodate these practicalproblems, the digital reference signals may be adjusted on the groundbefore flight and in flight via a respective radio channel.

If the axis A--A is inclined in the other direction to the horizon, thevoltage at the node falls.

The direct comparison of the light level signals S1 and S2 with thereference signal is appropriate when the axes A--A and B--B are alignedwith the pitch and roll axes of the aircraft. Especially in the formillustrated where the sensing unit has open holes 8 to direct the lightsensors, it may be found that a forwardly directed hole is subject toblocking, e.g. with engine oil. In this case it would be an advantage ifnone of the holes is aligned in the direction of flight.

To this end the axes A--A and B--B may be aligned at 45° to the pitchand roll axes of the aircraft, i.e. in directions parallel to thebisectors of the angles between the pitch and roll axes. With thisarrangement, the switches 24 are operated to their alternative positionsso that the signals input to the subtractors 26 result from an adder 28and a subtractor 30. The adder 28 produces a signal having a value onehalf of the sum of S1 and S2. The sum signal is input to the respectivesubtractor 26 to produce the signal R which represents the attitude ofthe aircraft about the roll axis. The subtractor 30 produces a signalwhich has a value one half the difference between S1 and S2. Thedifference signal is input to the respective subtractor 26 to producethe signal P which represents the attitude of the aircraft about thepitch axis.

The pitch and roll representative signals P and R, are dynamicallyshaped by respective differentiators 32 and adders 34 which add adifferental component to produce proportional/differential stabilisingsignals ##EQU1## where A is gain.

The model is controlled by radio signals received by a receiver 36. Thereceiver decodes the received signals and places pulse width modulatedsignals on output lines 38 to 46, one for each channel. Position demandsignals Pd on line 38, Rd1 on line 42 and Rd2 on line 46 are intended tocontrol the position of respective servos 48, 50 and 52 which in turncontrol the aircraft's elevator (controlled by servo 48) and ailerons(controlled one each by servos 50 and 52) to control the attitude of theaircraft. Separate controls for each aileron allow their use also asflaps. Conventionally the output lines 38 and 42 and 46 would beconnected direct to the respective servo. As shown in FIG. 2, however,the lines are connected to the controller 19 which converts the pulsewidth modulated signals to digital signals in convertors 54.

The stabilising signals ##EQU2## may be adjusted, as hereinafterdescribed, by gain setting multipliers 60 and by invertors 62 undercontrol of switches 64, and the adjusted signals are applied to themixers 56.

Mixers 56 act, broadly, to modify the digital demand signals Pd, Rd1 andRd2 increasing or reducing them in a sense to reduce the value of therespective stabilising signals ##EQU3##

The modified demand signals Pm, Rm1 and Rm2 are converted to pulse widthmodulated signals by convertors 58 and drive the respective servos 48,50 and 52.

In use, if the control sticks on the transmitter are neutralised, sothat in the prior art arrangements the aircraft would continue doingwhat it had been doing, the modification to the pulses produced by thecontroller 19 will return the aircraft to a level attitude in both axesand the right way up. However, if the mixing function is mere summation,every demand input intended by the pilot will be resisted orcounteracted by the stabilising signals.

To improve the response of the system to pilot inputs without reducingthe effectiveness of the stabilisation system if the pilot loosescontrol, the mixing function of the mixers 56 broadly reduces the effectof the stabilisation signals in dependence on increasing values ofelevator and aileron position demand signals respectively.

Broadly, for each mixer:

    U.sub.output =U.sub.input1 +U.sub.input2 +F(U.sub.input1,U.sub.input2,P)

where: U_(output) is the output signal from the mixer

U_(input1) is the stabilisation signal appearing at one input to themixer

U_(input2) is the pilot demand signal appearing at the other input tothe mixer

P is a parameter defining the mixing law and which may be preset orderived from a further channel output of the receiver 36 on one of lines40, or 44.

The effect of the stabilisation signal may be reduced proportionally,for example:

    F(U.sub.input1,U.sub.input2,P)=-{U.sub.input1 *(U.sub.input2 /maxU.sub.input2)}

where max U_(input2))} is the value of U_(input2))} which gives maximumcontrol deflection.

In another example, the function may be defined as follows:

    F(U.sub.input1,U.sub.input2,P)=0 if U.sub.input2 <maxU.sub.input2 /2

    F(U.sub.input1,U.sub.input2,P)=(a negative sign)U.sub.input1 if U.sub.input2 >maxU.sub.input2 /2

The gain set by the multipliers 60 may be derived from an on boardpotentiometer 66 via analog to digital convertor 68, or from a signaltransmitted over a further channel (not shown).

If the aircraft is inverted, the modifications to the pulse widthmodulated signals will be in the wrong sense to return it to an invertedlevel attitude so it flies back the right way up.

The sensor unit may be mounted above or below the aircraft. However theeffect of the control unit 19 would need to be reversed according towhere the sensor unit is mounted. Supposing the sensor extends the servopulse width if the sensor unit is mounted above the aircraft in theorientation illustrated in FIG. 1, then it would need to produce areduction in pulse width if the sensor were mounted below the aircraftinverted compared with FIG. 1. Switches 64 and invertors 62 are providedorder to preset the controller to accommodate either orientation of thesensor unit.

The functions of the signal processing means may be convenientlyimplemented using a programmed microprocessor.

In alternative arrangements, the stabilisation signals may be derivedfrom conventional gyros.

We claim:
 1. A radio controlled aircraft having an elevator and anaileron and including a radio receiver for receiving an elevatorposition demand signal for specifying a required position for theelevator and at least one aileron position demand signal for specifyingrequired positions for the aileron; stabiliser means for generatingpitch and roll stabilisation signals having values dependent ondifferences in the aircraft's attitude from level in pitch and roll,respectively, and being effective only for changing the attitude of theaircraft to level, mixer means for combining the pitch and rollstabilisation signals with the elevator and aileron position demandsignals, respectively, control means for controlling the positions ofthe elevator and the aileron in accordance with respective combinedsignals, and said mixer means being arranged to combine the pitch androll stabilisation signals with the elevator and aileron position demandsignals in accordance with a function which reduces the effects of thestabilisation pitch and roll signals in said combined signals independence on increasing values of elevator and aileron position demandsignals, respectively.
 2. A radio controlled aircraft as claimed inclaim 1 wherein, in accordance with said function, the combined signalscomprise the sum of the pitch and roll stabilisation signals with theelevator and aileron position demand signals, respectively, when theelevator and aileron position demands signals are below respectivepreselected levels, and are equal to the elevator and aileron demandsignals, respectively, when these are above said preselected levels. 3.A radio controlled aircraft as claimed in claim 1 wherein the values ofthe pitch and roll stabilisation signals in said combined signals arereduced in proportion to any increase in the respective elevator andaileron position demand signals from maximum values, when the respectiveposition demand signals are zero, to zero, when the respective positiondemand signals have predetermined values larger than zero.
 4. A radiocontrolled aircraft as claimed in claim 1, wherein the radio receiver isadapted for receiving a gain setting signal for modifying the values ofthe pitch and roll stabilisation signals prior to their bring combinedwith said demand signals.
 5. A radio controlled aircraft as claimed inclaim 1, including two pairs of directional light sensors, the sensorsof each pair being responsive to light input from two differentdirections on opposite sides of a respective axis to provide respectivelight level signals indicative of the levels of light sensed, saidstabiliser means being responsive to light level signals from each saidpair of sensors to provide two light level difference signals.
 6. Aradio controlled aircraft as claimed in claim 5, wherein the lightsensors are photo conductive light sensors connected in series across areference voltage.
 7. A radio controlled aircraft as claimed in claim 6,wherein the light sensors are aligned approximately normal to respectivepitch and roll axes.
 8. A radio controlled aircraft as claimed in claim6, wherein the light sensors are aligned in directions approximatelybisecting the angles between the pitch and roll axes.
 9. A radiocontrolled aircraft as claimed in claim 8, including means to add thetwo light level difference signals in order to provide one of the pitchand roll stabilisation signals and to subtract one light leveldifference signal from the other in order to provide the other of thepitch and roll stabilisation signals.